Less traffic is crossing the Blue Water Bridge, but why that is depends on who you ask.

The Bridge and Tunnel Operators Association released data this week for passenger vehicles, trucks and buses and other miscellaneous traffic at close to a dozen structures across the country.

While some bridge or tunnels saw small upticks in numbers, such as at Detroit’s Ambassador Bridge, others saw a small decline in traffic in 2017 from the year before.

At the Blue Water Bridge, traffic overall declined by nearly 1 percent. The bridge, according to BTOA statistics, saw traffic totals for 2016 and 2017 that were both in the 4.6 million range ­— but down 10,990 in cars, 31,718 in trucks, and 43,051 vehicles overall.

But that small downtick is pretty much on trend with traffic across the board, according to BTOA President Ron Rienas. Up or down, he said the changes are minor.

“It’s basically flat,” he said. “… In so far as the declines for the Blue Water Bridge, generally, (it’s) not that significant with less than 2 percent of trucks and less than a half percent for cars, but it’s kind of keeping in what the trend has been since 2000, which is decline.”

The Michigan Department of Transportation makes bridge traffic data available on its website from the late 1930s into the 2000s. According to the data, traffic overall peaked at 6.1 million in 1991 — fluctuating slightly through the 1990s before taking hits following Sept. 11, 2001, and the 2008 economic recession.

“Just referencing our crossings totals from 2008 on, the numbers move up and down, but no changes that would be considered alarming,” she said in an email. “It could be something as simple as construction or maintenance that impacts traffic at other crossings, making it easier for motorists and truckers to take a different route. BWB totals may climb when other routes are impacted by construction, either on or approaching the crossings, and vice versa.”

That latter effect was something BTOA suggested was in play for Detroit’s two border crossings.

The Detroit-Windsor tunnel was closed for 10 days in late October and saw other partial closures overnight through December. In turn, the tunnel saw a 3.7 percent decline in traffic while the Ambassador Bridge saw a 1.8 percent increase.

Traffic at every included border crossing has seen larger declines when compared to 2000 traffic.

The Blue Water Bridge has seen 22.5 percent less traffic between 2017 and 2000, while Detroit’s bridge and tunnel have seen 43.7 percent and 50.9 percent declines, respectively.

Officials said some of the reasons are fairly obvious.

In 2000, Homeland Security as a federal department didn’t exist, and people weren’t required to have an enhanced license or passport to cross the border.

“Of course, that has an effect,” said Ken Hammond, a public liaison for U.S. Customs and Border Patrol. “I don’t know how far that effect was, but I’m also certain security enhancements would be a factor.”

In 2009, he said the Western Hemisphere Travel Initiative required specific documentation to cross borders. But they’ve also since installed radio-frequency identification technology, he said, as well as trusted trader and traveler programs to help expedite people’s experience crossing the border.

But Rienas, who’s also manager of the Peace Bridge in Buffalo, N.Y., said there may be other reasons traffic has taken a hit overall since 2000.

For just passenger cars, 43.6 million crossed in 2000 at the 11 BTOA structures versus the 26.8 million that did 17 years later — a 38 percent decrease. Rienas attributed those to greater security provisions, including required ID, and related longer delays at the border.

For trucks, which saw a 20 percent decrease overall, he said, “That’s a different kind of question,” attributing much of that drop to the shift in manufacturing to different areas of the country.

In 2000, Rienas said Canada was still a “number one trading partner” for the U.S. “That’s no longer the case,” he said. “Canada is now number three behind China and Mexico in terms of exports to the U.S.”

Despite that, however, the Blue Water Bridge was one of two to see a positive change in long-term truck traffic with a 4.55 percent increase between 2000 and 2017.

Contact Jackie Smith at (810) 989-6270 or jssmith@gannett.com. Follow her on Twitter @Jackie20Smith.